ASCOT, England (Reuters) - British police found no sign of radiation or chemicals during searches of former Russian oligarch Boris Berezovsky's home near London on Sunday and said there was no evidence anyone else was involved in his still unexplained death.

Once known as the "godfather of the Kremlin", the former billionaire powerbroker helped Vladimir Putin rise to the top before falling out of favour himself and fleeing in 2000 to Britain where he became one of the president's biggest critics.

A day after the 67-year-old's body was found in the locked bathroom of his imposing country mansion, police said it was still unclear how he had died.

Senior officers called in radioactive, biological and chemical experts after a paramedic's personal radiation alarm was set off at the house after Berezovsky was declared dead on Saturday. However, specialist searches found no dangerous substances.

Berezovsky was a friend of Alexander Litvinenko, the former Russian spy who was poisoned with radioactive material in London in 2006, a murder that strained diplomatic ties between Britain and Russia.

"It would be wrong to speculate on the cause of death until the post mortem has been carried out," said Detective Chief Inspector Kevin Brown, deputy senior investigating officer. "We do not have any evidence at this stage to suggest third party involvement."

The interior ministry will conduct an autopsy to try to establish the cause of death, although that is unlikely to take place on Sunday, police added.

ASSASSINATION PLOTS

Berezovsky had survived assassination attempts, including a bombing that decapitated his driver, and said he feared for his life after he repeatedly denounced Putin and called for him to be forced from office.

However, friends said Berezovsky, once the personification of the ruthless post-Soviet era of a small group obtaining massive wealth and political power, had been depressed, had lost his fortune and may have committed suicide.

The Russian edition of Forbes magazine quoted Berezovsky as saying less than 24 hours before he was found dead that he lamented his exile and had lost sight of the "point of life".

"I do not know what to do. I am 67 years old. And I do not know what to do next," it quoted him as saying in an interview in the Four Seasons restaurant, which it said it conducted off the record but was publishing anyway because of his death.

"I've ... lost the point," Berezovsky said.

"Of life?" asked the interviewer.

"The point of life," Berezovsky replied.

Police stood guard outside Berezovsky's mansion, a French-style property with a swimming pool and lake in Ascot, a few miles from Queen Elizabeth's Windsor Castle, 25 miles (40 km) west of London. Inside, detectives were carrying out a thorough search of the house.

Giving the first official details of Berezovsky's final hours, police said a concerned employee called the ambulance service at 1523 GMT on Saturday because he was worried that he hadn't seen Berezovsky since around 2230 GMT on Friday.

"The employee said that he forced open the bathroom door which was locked from the inside and discovered the body of Mr Berezovsky on the floor," police said. No one else was in the house when the emergency services arrived.

Detectives are speaking to Berezovsky's family and friends to "gain a better understanding of his state of mind", the police statement added.

Berezovsky was under pressure after losing a $6 billion court case to Chelsea Football Club owner Roman Abramovich, a former business partner he sued in one of the most expensive cases in British legal history.

Putin's spokesman said Berezovsky, seen by Moscow as a criminal who should stand trial for fraud and tax evasion, had written to Putin asking for forgiveness - a suggestion dismissed by one of the oligarch's friends.

"Berezovsky sent Vladimir Putin a letter he wrote personally, in which he acknowledged that he had made many mistakes, asked Putin's forgiveness for these mistakes and appealed to Putin to help him return to his homeland," said Putin's spokesman Dmitry Peskov.

A friend of Berezovsky's in London, Andrei Sidelnikov, told Reuters the idea that the businessman would write a letter to Putin was "complete nonsense".

"He was a sane person and he understood that he would never be able to return under Putin's regime, for political reasons," Sidelnikov said.

A former mathematician, Berezovsky made millions running post-Soviet car dealerships and expanded his business empire in the 1990s to encompass the ticket revenue of the flagship airline Aeroflot and control of the one of the main state television stations, which he openly used for political aims.

He was one of a handful of politically savvy businessmen who became instant billionaires under former president Boris Yeltsin when the state arranged for them to buy giant oil companies for what quickly proved to be a fraction of their value.

As one of the central figures in Yeltsin's inner circle, he helped forge the career of Yeltsin's hand-picked successor Putin, a little-known official named prime minister in 1999 and acting president when Yeltsin resigned on millennium eve.

After Putin was confirmed in the presidency in an election in 2000, Berezovsky quickly fell out with him and left for Britain where he denounced his former ally as a corrupt "bandit" surrounded by former KGB agents.

Berezovsky was humiliated last year when he lost his legal battle with Abramovich over shares in Russia's fourth biggest oil company.

Some associates said he had struggled with the cost of losing, estimated at the time as more than $100 million. Berezovsky had kept a low profile since the defeat.

In another financial blow, he agreed to pay one of Britain's biggest-ever divorce settlements to his former wife Galina in 2011. Local media said the settlement was believed to be more than $100 million.

"He had no money, he had lost it all. He was unbelievably depressed," Tim Bell, a public relations executive who was one of his closest British advisers, told the Sunday Times newspaper. "It's all very sad."

Writing by Peter Griffiths in London; Additional reporting by Maria Golovnina and Guy Faulconbridge in London and Thomas Grove, Maria Tsvetkova, Alexei Anishchuk in Moscow; Editing by Peter Graff